The use of highly reflective “cool” roofing materials can save energy used for air conditioning, mitigate urban heat island effects, and improve urban air quality. Replacing a conventional dark roof with a solar-reflective cool roof can reduce a building's annual air conditioning energy use by anywhere from 5% to 20%. Widespread use of cool roofs, by also lowering local outside air temperatures, can further decrease air conditioning energy use by another 10% and reduce the temperature-dependent rate of smog formation. Solar-reflective envelope surfaces can also help contribute to the slowing of global warming. Replacing 10 m2 of dark roofing with white roofing induces a negative radiative forcing in the global atmospheric energy balance sufficient to offset emission of over 2 tonnes of CO2. In all urban areas in hot and temperate regions of the world, the expected emitted CO2 offset for a plausible 0.25 increase in the albedo (i.e., solar reflectance: the ratio of reflected to incident sunlight) of roofs was estimated to be about 70 Gt (gigatonnes) of CO2. Considering that the present annual global CO2 emission is around 30 Gt, installing cool roofs could offset over 2 years of global CO2 emission.
The solar absorptances of building envelope surfaces are often increased by soiling and/or weathering processes, including but not limited to deposition of soot, microbiological growth, and chemical or physical changes induced by exposure to ultraviolet (UV) radiation and outdoor elements. This can increase the need for interior air conditioning by making initially “cool” surfaces hot. It has been found that highly reflective cool roofs lose up to 40% of their initial solar reflectance over time.